1. Field of the Invention
The present invention relates to a process for producing very light moldings from polyolefin foam particles of high density.
2. Discussion of the Background
Processes for producing polyolefin foam particles are known. For example, in DE-A-21 55 775, a dispersion of polymer particles in a liquid dispersion medium is heat-treated under pressure. During this heat treatment, a part of the dispersion medium is dissolved with stirring in the polymer. After a treatment time of about 30 minutes, the reactor contents are discharged into a low-pressure chamber at ambient temperature. This results in expansion of the polymer particles to give foam beads. Particularly uniform foam qualities are supposed to be achieved by addition of organic or inorganic fillers to the polymer.
The foaming of ethylene/propylene random copolymers is described in EP-A-0 053 333 through the use of a system comprising polymer particles, water as dispersion medium, a solid dispersing aid and a volatile blowing agent.
Particles produced by dispersion or extrusion foaming can also be obtained in shrunken form. For example, DE-A-25 24 196 describes how the bulk density of light foam particles can be increased by shrinking. This is achieved by subsequent heating and pressing together, with the goal being to produce foamed articles having a high density.
EP-A-0 453 836 describes foam particles which, after expansion, shrink to as little as 40% of their original volume. These are post-treated under pressure and temperature to reverse the shrinkage, so that foam beads having a bulk density of 10 g/l are obtained. EP-A-0 453 836 thus teaches that shrunken foam beads cannot be directly used in the production of light moldings.
Similarly, DE-A-39 22 207 describes an undesired shrinking after the expansion. By means of post-treatment under pressure and at relatively high temperature, the foam beads regain their original volume.
Finally, JP-A-04/015 236 describes specific conditions which must be adhered to in the expansion process to obtain shrunken foam particles. As an example, shrunken foam particles having a bulk density of 25 g/l are obtained; on processing into moldings, a density of 23.1 g/l is achieved. Without influencing the foaming process, a bulk density of 16.3 g/l is obtained. From this, moldings having a density of 24.4 g/l can be produced. The advantage of this process is lower transport costs compared with the lighter material.
Thus it was known, on the one hand, that shrunken foam particles can be used directly for producing moldings. However, on the other hand, the quality of these moldings is described as unsatisfactory; in addition, the density of the moldings is restricted to values lying in the region of the bulk density of the shrunken foam particles.
In the prior art (EP-A-0 453 836; DE-A-39 22 207), these disadvantages are overcome by the shrinkage first being reversed by a further expansion of the shrunken particles. The particles thus obtained are then used for production of moldings. However, this process is not satisfactory since the additional expansion step is complicated and expensive. In addition, the densities of the moldings produced in this way are still too high for many purposes.